The present invention is directed to an electronic ballast for fluorescent lamps, and in particular electronic ballasts having an inverter that has it input side connected to an AC source via a series connection of a harmonic filter and of a rectifier. Such an electronic ballast has its output side connected to at least one load circuit composed of a series circuit of an inductor and a parallel circuit of a capacitor and a fluorescent lamp. An inverter in the electronic ballast is designed as a switch bridge arrangement having two switch branches and two capacitor branches whose bridge terminals which form the output of the inverter are formed, first, by the common junctions of the two switch branches and, second, by the two capacitor branches, whereby the two switch branches are composed of electronic switches having freewheeling diodes connected in parallel, these switches being opened and closed in push-pull fashion having a switching frequency that is high in comparison to the alternating frequency of the AC source.
A prior art electronic ballasts of this type are disclosed, for example, by the European reference EP 0 121 917 A1. The switch bridge arrangement used has only one capacitor branch. This, however, is only an economic structure of such a switch bridge arrangement as shown, for example, by the reference of C. H. Sturm, "Vorschaltgeraete und Schaltungen fuer Niederspannungs-Entladungslampen", Brown, Boveri & Cie AG, Mannheim 5th Edition, 1974, pages 343 and 344.
High-voltage electrolyte capacitors which are used in such electronic ballasts for smoothing the rectified line alternating current are designed for a direct voltage of 450 V and represent a standard that has been tested extensively. This electrical voltage of 450 V DC is completely adequate in view of a peak line voltage of 439 V that results from a line alternating voltage of 277 V plus or minus 12%. When, however, additional measures are taken for increasing the power factor, then either a high-voltage electrolyte capacitor having a significantly higher direct voltage tolerance or, two series-connected electrolyte capacitors must be utilized. The series connection of two electrolyte capacitors, however, also increases the costs of such an electronic ballast and also causes additional losses in view of the necessary compensation of leakage current.